Currently the setting for which categories' posts to show is a dropdown list where only one category can be selected. It would be most useful to have it as a list with checkboxes instead, allowing multiple choices.
This allows for example a front page like this:
First, layout 1, showing posts from the most important category X.
And below, layout 2, showing posts from categories Y and Z.
In reference to my other suggestion about excerpt option improvements, those two layouts could have completely different excerpt and other options.